Referring to FIG. 1, there is shown a motorized roller tube system 10 having a prior drive assembly 12. The motorized roller tube system 10 includes a rotatably supported roller tube 14 and a flexible member 16, such as a window shade fabric, windingly received by the roller tube 14. The flexible member 16 is typically engaged to the roller tube 14 by securing an end portion of the flexible member 16-to the roller tube 14. There are a variety of well-known means for securing the flexible member 16 to the roller tube 14 including, for example, the use of double-sided tape, or by a clip member received over an end portion of the flexible member 16 in a locking channel provided on the exterior of the roller tube 14. The roller tube 14 is driven in opposite rotational directions by the drive assembly 12 for winding and unwinding the flexible member 16 with respect to the roller tube 14. The prior drive assembly 12 includes an elongated housing 18 and a puck 20 located adjacent an end of the housing 18. The puck 20 engages an inner surface of the roller tube 14 to drive the roller tube 14 as the puck is rotated by the drive assembly 12.
The prior roller tube drive assembly 12 includes a motor 22 and gear assembly 24 located within an interior of the housing 18 and connected to the puck 20. The motor 22 and gear assembly 24 are shown in FIG. 2 removed from housing 18. The motor 22 of prior drive assembly 12 is a DC motor. Referring again to FIG. 1, the drive assembly 12 is received within the interior of the roller tube 14. For this reason, this type of roller tube drive assembly is referred to as an “internal” drive assembly. Other known motorized roller tube systems include drive assemblies that are located externally of the roller tube.
The motor 22 includes an output shaft 23 that is rotated by the motor at a rotational speed referred to herein as the “motor speed”. The prior drive assembly 12 operates the motor at a motor speed of approximately 2000 rpm. The gear assembly 24, which is connected to the output shaft of the motor 22, reduces rotational speed from the relatively fast speed of 2000 rpm input from motor 22 to a relatively slow output rotational speed of approximately 27 rpm for roller tube 14. The gear assembly 24 of the prior drive assembly 12, therefore, has a gear ratio of approximately 74:1 (i.e., 2000/27).
The torque capability of a motor varies depending on the motor speed. Therefore, the motor of any motorized roller tube system must provide a torque capability at the operating motor speed that is sufficient to wind the flexible member 16 onto the roller tube 14. Referring to FIG. 3, the performance characteristics for motor 22 of prior drive assembly 12 are shown graphically. Graphs of this type are referred to as “motor curves”. The relationship between motor speed (shown on the Y-axis) and motor torque capability (shown on the X-axis) is represented by line 26. As shown, the maximum motor speed for motor 22 is approximately 3150 rpm and the maximum motor torque capability is approximately 280 m-Nm. As also shown, the motor torque capability for DC motor 22 varies linearly throughout the entire range of motor speeds. In other words, the motor will provide increasing torque capability with decreasing motor speed even at very slow speeds approaching zero. It should be understood the motor torque values on speed/torque line 26 of FIG. 3 represent capability rather than fixed values of operating motor torque. In other words, the motor 22 is capable of operating at a given motor speed at any torque between zero (i.e., an unloaded condition) and the value represented on the speed/torque line 26. At the operating speed of 2000 rpm, the torque capability of motor 22 is approximately 99 m-Nm.
As shown in FIG. 3 by curve 28, the efficiency of motor 22 also varies depending on the motor speed. The efficiency, which is shown on the Y-axis with motor speed, is determined by reading vertically from the speed/torque line 26 to the efficiency curve 28. Thus, at the operating motor speed of 2000 rpm, the motor 22 of prior drive assembly 12 has an efficiency of approximately 25 percent. As shown, the motor efficiency of 25 percent is the peak efficiency for motor 22. The motor speed associated with peak efficiency is referred to herein as the peak efficiency motor speed. The peak efficiency motor speed represents approximately 65 percent of the maximum motor speed (i.e., 2000/3100).
Although the particular values of motor speed, torque capability, and efficiency will vary for different DC motors, there are certain characteristics that are shared by all DC motors. Firstly, motor speed and motor torque capability will vary linearly, and inversely, throughout the entire range of motor speeds including very low speeds approaching zero. Secondly, motor efficiency will generally reach peak efficiency under light-duty conditions (i.e., relatively low torque capability at a motor speed greater than 50 percent of maximum motor speed). Prior drive assemblies include motors configured and operated by the drive assembly under light-duty conditions near the peak efficiency motor speed. As described below in greater detail, operation of the motors under such relatively light-duty conditions is in accordance with motor manufacturer recommended operation of the motor.
The gear assemblies of known roller tube drive assemblies include planetary spur gears. Planetary spur gears are desirably economical in construction and provide efficient transmission compared to other types of gears. Spur gears, however, tend to be noisy in operation compared to other gear types because of sound generated as peripheral teeth contact each other. This contact sound associated with meshing teeth is sometimes referred to as “gear slapping” and increases as the rotational speed of the meshing gears is increased. Known gear assemblies also include gear stages having helical gears. Helical gears include elongated spiral flights that constantly engage with flights of other helical gears. The constant engagement of the flights eliminates the slapping noises associated with contact between the teeth of spur gears. Helical gears, however, tend to be less economical and less efficient than spur gears.
The gear assembly 24 of prior drive assembly 12 includes three gear stages 30, 32, 34. The gear assembly 24 is a hybrid gear system and includes a first stage 30 having helical gears and second and third stages 32, 34 each having planetary spur gears. The first gear stage 30 is located closest to the motor 22. The gears of stage 30, therefore, are rotated at the relatively fast motor speed of 2000 rpm. The rotational speed in the second and third stages 32, 34, however, is stepped down from the 2000 rpm motor speed. Thus, the hybrid construction of prior drive assembly 12 represents a trade-off in which quieter, less efficient, more expensive helical gears are used in the relatively fast first stage 30, while efficient, less expensive, but noisier, planetary spur gears are used in the relatively slower second and third stages 32, 34.
Summary of the Invention
According to present invention, a quiet drive assembly for a motorized roller tube system includes a motor and a gear assembly having multiple gear stages. The drive assembly is configured such that the motor is driven inefficiently at relatively slow motor speeds. Preferably, the operating motor speed is less than 50 percent of a maximum motor speed. Preferably, the motor is operated at an efficiency that is less than 50 percent of a peak efficiency for the motor. Preferably, the motor has a torque capability at the operating motor speed that is greater than 4 times the torque capability for the motor at the peak efficiency motor speed.
According to one embodiment, the motor is a DC motor and one or more of the stages of the gear assembly includes planetary spur gears. The quiet drive assembly preferably provides a sound pressure level during any movement of the roller tube of between approximately 40 dBA and 44 dBA within an ambient sound pressure level of approximately 38 dBA when measured at approximately 3 feet from the driven end of the roller tube. Sound pressure levels of this level are considered pleasant and non-distracting.
According to one embodiment, the gear assembly has a gear ratio of approximately 20:1 and the motor is driven at a motor speed between zero and 1500 rpm. Most preferably, the motor speed is approximately 850 rpm.
According to one embodiment, the motor is an AC motor. Preferably, the AC motor has 4 or less electrical poles. The AC motor includes an output shaft rotated at an operating speed between approximately 750 rpm and approximately 900 rpm.
According to one embodiment, the drive assembly is received within an interior of a roller tube having a diameter of less than 2 inches and the motor has a maximum motor torque capability of more than approximately 120 m-Nm.
Description of the Invention
Referring to the drawings, where like numerals identify like elements, there is shown in FIGS. 4 through 6 a roller tube drive assembly 40 according to the present invention including a motor 42 and a gear assembly 44 contained within an elongated housing 41. The drive assembly 40 of the present invention is adapted for receipt within a roller tube, such as the tube 14 of FIG. 1, to engage an inner surface of the roller tube for rotating the tube to wind or unwind a flexible member, such as a window shade fabric. The receipt and engagement of the drive assembly 40 is similar to that described above for the prior drive assembly 12. As described below in greater detail, however, the drive assembly 40 of the present invention is configured in a novel manner providing for reduction in roller tube diameter for driving a given applied load or, alternatively, driving a large applied load for a given roller tube diameter. Also, the novel configuration generates limited noise for relatively quiet roller tube movements while desirably utilizing spur gear transmission throughout the gear assembly 44.
The motor 42 of drive assembly 40 is preferably a DC motor. Motor 42 has an output shaft 43 for transmission of mechanical power at a motor speed and torque. DC motors are highly reliable, relatively inexpensive and possess adequate torque capability in sufficiently small sizes for most roller tube applications. DC motors include brushed and brushless DC motors. Brushed and brushless DC motors have similar torque/speed curves. Brushless DC motors, however, have a wound stator surrounding a permanent-magnet rotor, which is an inverse arrangement to that of a brushed DC motor. The construction of the brushless motor eliminates the need for motor brushes, which allow current to flow through the wound rotor in a brushed motor. The stator windings of a brushless DC motor are commutated electronically requiring control electronics to control current flow. Brushed DC motors are presently readily available in large varieties and, therefore, are presently preferred for economic reasons.
The majority of the noise generated by drive assembly 40 is created by motor 42 and by the gears in the gear assembly 44. These noise generating elements are shown in FIG. 5 removed from the rest of the drive assembly 40 to facilitate comparison with the corresponding elements of the prior drive assembly 12 of FIG. 2. The gear assembly 44 of drive assembly 40 includes first and second gear stages 46, 48 for reducing rotational speed from the rotational speed of motor 42 to the rotational speed desired for rotating a roller tube in which the drive assembly 40 is received. The gears in each of the stages 46, 48 of gear assembly 44 are planetary spur gears. As described above, the use of planetary spur gears throughout all stages of the gear assembly 44 is desirable because spur gears are economical and provide efficient gear transmission compared to other types of gears such as the helical gears in the first stage of prior drive assembly 12. The planetary spur gears of gear assembly 44 are preferably made from plastic.
Referring to FIG. 7, the motor curve for motor 42 is shown. Similar to the motor curve of FIG. 3 for motor 22, FIG. 7 graphically illustrates various performance characteristics for motor 42 including motor speed, motor torque capability and motor efficiency. As shown by line 51, the motor speed and motor torque capability for motor 42, like those of motor 22, are inversely proportional to each other throughout the entire range of motor speeds including very slow speeds approaching zero. The maximum motor speed for motor 42 is approximately 4200 rpm and the maximum motor torque capability is approximately 122 m-Nm. As shown by efficiency curve 53, the motor efficiency for motor 42 reaches a peak of approximately 75 percent when the motor is operated at a speed of approximately 3700 rpm.
The motor curve of FIG. 7 includes a manufacturer's recommended operating range, which is shown by shaded area 55. As shown, the manufacturer's recommended operating range for motor 42 includes motor speeds corresponding to relatively light-duty conditions (i.e., relatively high speeds and relatively low motor torque). Not surprisingly, the manufacturer's recommended operating range includes the peak efficiency motor speed of 3700 rpm. As discussed above, the motors of prior roller tube drive assemblies are operated by the drive assemblies under light-duty conditions in accordance with the manufacturer's recommendations. Specifically, the manufacturer for motor 42 recommends that the motor be operated at motor speeds above approximately 3200 rpm, which represents speed ranging between approximately 76 percent and 100 percent of the maximum motor speed for motor 42, which is 4200 rpm. Also similar to motor 18, the recommended operating range for motor 42 includes the peak efficiency motor speed of 3700 rpm.
Operating the motor of a roller tube drive assembly within the manufacturer's recommended range in conformance with established convention in the art would appear to be intuitively preferred. As discussed above, the recommended operating range includes the peak efficiency motor speed. Therefore, operation of the motor in the recommended range results in efficient operation of the motor. Also, the relatively light-duty conditions (i.e., relatively low torques) associated with the recommended range serves to limit overheating damage that could result from heavy-duty operation of the motor, thereby promoting motor life.
The drive assembly 40, however, is not configured to operate the motor 42 in the manufacturer's recommended range in conformance with established convention. Instead, the motor 42 of drive assembly 40 is preferably operated under heavy-duty conditions (i.e., relatively high torque) in a range of motor speeds represented in FIG. 7 by shaded area 57. As shown, the preferred operating range 57 includes motor speeds between 0 rpm and approximately 1500 rpm. The upper end of 1500 rpm for the preferred operating range represents approximately 36 percent of the maximum motor speed of 4200 rpm for motor 42. Most preferably, the drive assembly 40 operates the motor 42 at a speed of approximately 850 rpm, which represents only approximately 20 percent of the maximum speed. As shown by line 51 of FIG. 7, the motor torque capability for motor 42 when operated at a speed of 850 rpm is approximately 98 m-Nm. As shown by curve 53, the motor efficiency for motor 42 is approximately 19 percent when the motor is operating at the preferred speed of 850 rpm. This motor efficiency represents only approximately one-fourth of the peak efficiency for motor 42 (i.e., 19/75). The drive assembly 40 of the present invention is configured to operate the motor 42 at a motor speed that is well outside the recommended range under conditions that are very inefficient for the motor.
The torque capability of 98 m-Nm provided by motor 42 at its operating motor speed of 850 rpm is roughly equivalent to the 99 m-Nm provided by motor 22 of prior drive assembly 12 at its operating motor speed of 2000 rpm. However, the diameter of motor 22 is 1.65 inches while the diameter of motor 42 is only approximately 1.22 inches. The present invention, therefore, by operating inefficiently outside of the recommended operating range, provides similar torque capability for driving similar applied loads while allowing for reduction in the diameter of the motor. By reducing motor diameter, a corresponding reduction in the required roller tube diameter is provided. Limiting the roller tube diameter is desired aesthetically to avoid an installation that is bulky in appearance. It should be understood that, instead of decreasing motor diameter, the present invention could be used to increase torque capability for a given motor for increasing the applied load that is driven by the motor.
The motor 22 of prior drive assembly 12 has a length of approximately 2.7 inches. The aspect ratio (i.e., length/diameter) of motor 22, therefore, is approximately 1.64 (i.e., 2.7/1.65). This aspect ratio is typical for standard torque motors. Motor 42 of the present drive assembly 40 also has a length of approximately 2.7 inches. The aspect ratio of motor 42, therefore, is approximately 2.21 (i.e., 2.7/1.22). The effect of this increase in the aspect ratio of motor 42 can be seen by comparing FIGS. 2 and 5. It is known that torque capability for a motor varies in proportion to BID2L, where B is magnetic flux, I is current, and D and L are respectively diameter and length of the motor. Thus, the motor torque capability can be increased by increasing any one of B, I, D or L. Because the aspect ratio has been increased from that which is associated with standard torque motors, the motor 42 of the present drive assembly is considered a “high” torque motor. The increased torque capability for motor 42 provided by increased aspect ratio (i.e., increased length) partially offsets the decreased torque capability associated with the decreased diameter. Of course, the reduction in diameter has a much greater impact on torque capability than the increase in length because the diameter is squared in the above relationship (i.e., BID2L). The present invention, therefore, also provides for increase in torque capability by operating the smaller diameter motor under the above-described heavy-duty conditions associated with the preferred range 57.
As described above, the torque capability of 98 m-Nm provided by motor 42 at its operating motor speed of 850 rpm is roughly equivalent to the 99 m-Nm provided by motor 22 of prior drive assembly 12 at its operating motor speed of 2000 rpm. The present invention, however, is not limited to any particular torque capability. It is conceivable, therefore, that the drive system could be configured to include a smaller diameter motor having a reduced torque capability compared to motor 42 for use within a smaller diameter roller tube. For example, a motor having a maximum torque capability between 50 m-Nm and 75 m-Nm could be used to drive a roller tube having a diameter less than approximately 1.625 inches.
As discussed above, planetary spur gears are a preferred gear type because of their economy and their gear efficiency but also tend to be undesirably noisy when driven at the relatively high rotational motor speeds associated with prior art drive assemblies. By reducing the motor speed to approximately 850 rpm, however, the present invention desirably allows for the use of spur gears in each stage of the gear assembly 44 without excessive noise being generated in the first stage 46 from gear slapping. As discussed above, the reduction in motor speed to 850 rpm also reduced the gear ratio required by gear assembly 44 to approximately 20:1. As a result, it was possible to reduce the number of gear stages from three to two. Such a reduction in the number of stages provides for a reduction in the total number of gears in the assembly thereby further reducing the noise generated by the gear assembly.
It is desirable that the drive assembly of a motorized roller tube system is capable of variable speed control of the drive assembly motor. Such variable speed control is desirable to account for changes in the effective winding radius for substantially constant movement of a flexible member being wound onto the roller tube. As a flexible member is wound onto a tube, the flexible member forms layers (or “windings”) such that the effective radius at which the flexible member is received by, or delivered from, the roller tube changes. Thus, if a roller tube were to be driven at a constant rotational speed, the speed at which the flexible member is moved (sometimes referred to as the “linear speed” or the “fabric speed”) would vary because of change in the effective winding radius. It should be understood that rotational speed will need to be reduced as the flexible member is wound onto a tube in order to maintain a constant fabric speed and, therefore, that the rotational speed will be greatest when the roller tube is being driven at or near the point at which the flexible member is fully unwound from the roller tube (i.e., a “fully-lowered” or “fully-closed” position). Also, the least amount of material is wound onto the tube when the flexible member is at the fully-lowered position of the flexible member such that the flexible member provides the least amount of sound attenuation for the roller tube in this position. The sound level produced by the motorized roller tube system, therefore, is greatest when the drive assembly is driving the roller tube at or near the fully-lowered position of the flexible member.
The present invention provides a drive assembly 40 that desirably includes spur gears in each stage of its gear assembly 44 while also limiting noise that is generated by the drive assembly. A motorized roller tube system including the drive assembly 40 housed within a 1.625 inch diameter roller tube was used to drive a typical applied load of approximately 8.1 in-lb (i.e., a 10 pound flexible member applied at 0.81 inch radius). Sound levels generated by the motorized roller tube system were measured using a sound pressure meter at a distance of approximately 3 feet from the driven end of the roller tube. The sound pressure level produced by the motorized roller tube system in an ambient of approximately 38 dBA when the drive assembly 40 is driving the roller tube at or near the fully-lowered position of the flexible member (i.e., the maximum sound level produced by the motorized shade assembly) is approximately 43 dBA. An ambient level of 38 dBA is a sound pressure level in a relatively quiet office setting such as a private office with the door closed, for example. A sound pressure level of between approximately 40-44 dBA generated by a motorized roller tube system in such a setting is considered non-distracting and even pleasant. The sound level generated by the present drive assembly having spur gears driven at rotational speeds well below the speeds associated with the motor manufacturer's recommended operating range compares favorably with that of prior motorized roller tube systems having spur gears driven at the faster rotational speeds recommended for the motor. Such motorized roller tube systems include systems generating sound pressure levels exceeding 50 dBA at approximately 3 feet in an ambient of approximately 38 dBA. Sound pressure levels exceeding 50 dBA in such an ambient environment are considered distracting and even annoying.
The above-described gear assembly 44 includes two gear stages 46, 48. The number of gear stages, however, is not critical. A drive assembly according to the present invention, therefore, could include more than the two stages that are shown in the above-described embodiment. As discussed above, however, reducing the number of gear stages desirably provides for reduction in the total number of gears in the gear assembly and, accordingly, a reduction in gear slapping noise.
As discussed above, inefficient operation of the motor 42 by drive assembly 40 under heavy-duty conditions is counter-intuitive. In addition to inefficient operation of the motor, sustained operation of a motor under the heavy-duty torque conditions associated with the preferred operation range 57 could overheat the motor potentially causing life-shortening damage of the motor. The motors of motorized roller tube systems, however, are not ordinarily operated in a continuous fashion. In a typical motorized roller tube system, such as a window shade for example, the shade fabric might be raised in the morning, lowered at night, and possibly adjusted to a number of other positions at infrequent intervals during the day. Therefore, except in the most unusual situations, the inefficient operation of drive motor 42 will not appreciably effect the motor in terms of longevity. To protect the motor 42, however, it is conceived that the drive assembly 40 could be configured to track the run time of motor 42. The motor 42 could then be disabled in the event that excessive run time has occurred during a given period of time that could adversely affect the motor if the motor were otherwise permitted to continue running. Alternatively, the condition of the motor could be monitored based on the temperature of the motor or related components, or the temperature of surrounding areas, using thermal-couples, thermistors, temperature sensors, or other suitable sensing devices.
Referring again to FIG. 4, some additional details of the construction of drive assembly 40 will now be discussed. The elongated housing 41 is tubular defining an interior in which the drive motor 42 and gear assembly 44 are housed. The drive assembly 40 preferably includes an electronic drive unit (“EDU”) 50 for controlling the operation of the drive motor 42. The EDU controller 50 includes a printed circuit board 52 for mounting control circuitry (not shown) of the controller 50. The controller 50 could be configured to track run time of the motor 42 in the above-described manner and to disable the operation of motor 42 in the event that overuse of the motor 42 within a given period of time could damage the motor 42. The EDU controller 50 includes a bearing sleeve 54 and bearing mandrels 56 adjacent an end of the housing 41. Electronic drive units for motorized roller tube systems are known and no further description is necessary.
The drive assembly 40 includes a drive puck 58 located adjacent an end of the housing 41 opposite the EDU bearing sleeve 54 and mandrels 56. The drive puck 58 is connected to a puck shaft 60 that is rotatably supported with respect to the housing 41 of drive assembly 40 by a drive bearing 62. The puck shaft 60 is connected to the gear assembly 44 of drive assembly 40 such that actuation of the drive motor 42 drivingly rotates the drive puck 58. The drive puck 58 includes longitudinal grooves in an outer periphery to promote engagement between the outer surface of the puck 58 and an inner surface of a roller tube when the drive assembly is received within a roller tube. The drive assembly 40 is adapted for receipt within the interior of a roller tube such that the EDU bearing sleeve 54 and mandrels 56 are located adjacent an end of the roller tube. The drive assembly 40 also includes brake 64 having a brake input 66, a brake output 68 and a brake mandrel 70. The brake 64 defines an interior in which the puck shaft 60 is received. The brake 64 is adapted to engage the puck shaft 60 to prevent relative rotation between the motor 42 and the drive puck 58. The engagement of the brake 64 prevents a flexible member from unwinding because of load applied to a roller tube by an unwound portion of the flexible member and any hem bar carried by the member, thereby holding the flexible member in a selected position. Brakes for roller tube drive assemblies are known and no further description is necessary.
Referring to FIG. 6, an embodiment of the motor 42 and gear assembly 44 of drive assembly 40 is shown in greater detail. The gear assembly 44 includes a ring gear 72 received within an interior of a ring gear cover 74. A motor adapter 76 is located between the motor 42 and the ring gear cover 74 and engages an end of the ring gear cover 74. The ring gear cover 74 includes a tab 78 received by a correspondingly shaped notch 80 of the motor adapter 76 to limit relative rotation therebetween. The ring gear cover 74 also includes an end fitting 82 received by the brake mandrel 70.
The gear assembly 44 includes a sun gear 45 that is attached to the output shaft 43 of motor 42 such that the sun gear 45 rotates with the output shaft 43. Preferably, the sun gear 45 is pressed onto the output shaft 43. Each of the first and second stages 46, 48 of gear assembly 44 includes three planetary spur gears that meshingly engage longitudinal teeth 96 formed on an inner surface of the ring gear 72. The sun gear 45 meshingly engages the spur gears of the first stage 46 such that the spur gears of the first stage 46 are rotated by the sun gear 45 at the motor speed. The spur gears of the first stage 46 are rotatingly received on pins 90 of a sun carrier 88. The spur gears of the second stage 48 are rotatingly received on pins 94 of a hex carrier 92. A sun gear 98 is fixed to the sun carrier 88 opposite the pins 90 and meshingly engages the spur gears of the second stage 48 to rotate the second stage gears as the sun carrier 88 is driven by the first stage 46. A hex socket 100 is fixed to the hex carrier 92 opposite the pins 94. The gear assembly 44 also includes a second stage adapter 102 including a hex head 104 received by the hex socket 100 of the hex carrier 92 and a socket 106 opposite the hex head 104 receiving an end of the drive puck shaft 60. The second stage adapter 102 transfers rotation from the hex carrier 92 to the drive puck 58 as the hex carrier 92 is driven by the second stage 48.
The controller 50 of drive assembly 40 preferably provides variable-speed control of the motor speed of motor 42. Such variable-speed control is desirable in a roller tube drive assembly for speed adjustments to account for winding of the flexible member onto the roller tube such that the movement of the flexible member (referred to as “linear speed” or “fabric speed”) is substantially constant. An example of such a control system is disclosed in U.S. patent application Ser. No. 10/774,919. filed Feb. 9, 2004. entitled “Control System for Uniform Movement of Multiple Roller Shades”, which is incorporated herein by reference in its entirety.
As the flexible member is wound onto the roller tube, the material of the flexible member is formed into layers (or “windings”). The layering of the fabric changes the radius at which the fabric is received by, or delivered from, the roller tube. Thus, if the roller tube is driven at a constant rotational speed, the speed of the flexible member will tend to increase as the member is being wound onto the roller tube. It is known to control motor speed for a DC motor by controlling the voltage to the motor using pulse-width modulation. An example of a motorized roller tube system using pulse-width modulation for variable motor speed is disclosed in U.S. Pat. No. 5,848,634. which is incorporated herein by reference.
The motor 42 of the above-described drive assembly is a DC motor, preferably a brushed DC motor. There may be applications, particularly when the applied load to be driven by the motor is relatively large, where an AC induction motor may be preferred over a DC motor. Such a situation could arise, for example, where a single motor is driving multiple roller tubes arranged in end-to-end fashion. For variable-speed control using an AC induction motor, the frequency and the applied voltage to the motor are modulated instead of just the voltage. An AC induction motor is typically wound with a set of stator windings, each driven with an AC voltage waveform. Typically, there are three separate windings spaced about the periphery of the motor stator to be driven by three phases of an AC voltage waveform. The phase displacements of the drive voltage waveforms sets up a rotating field in the rotor section of the motor. The reaction between the induced fields in the rotor and the fields in the stator creates a net torque on the rotor. The speed at which the rotor turns is related to the frequency of the drive waveform and the number of electrical poles created by the winding structure of stator. This relationship is stated in the following equation: n=120×F/P, where n is the rotor speed in rpm, F is drive voltage frequency in Hertz, and P is the number of electrical poles.
Commercially available AC induction motors typically include 2 or 4 poles. This configuration facilitates manufacture of stator windings. AC induction motors having 2 poles and 4 poles will typically run at nominal speeds of 3600 rpm and 1800 rpm, respectively, when driven with a 60 Hz drive voltage waveform. To operate these type of motors at speeds of about 750 to 900 rpm, a reduction of operating frequency is required. This is accomplished with a frequency controlled inverter circuit. By way of example, a 4 pole AC induction motor will need to be operated with a drive frequency of about 25 Hz to run at a rotor speed of about 750 rpm.
As described above, the drive assembly 40 of the present invention is adapted for receipt within a rotatably supported roller tube, such as the roller tube 14 depicted in FIG. 1. It should be understood, however, that the present invention is not limited to use within cylindrical tubes. The rotatably supported tube, therefore, could be any elongated member capable of being rotatably supported and adapted for winding receipt of a flexible member. Therefore, the roller tube could have a non-circular cross section such as hexagonal or octagonal for example. The non-circular cross section could also conceivably be a non-symmetrical shape such as an oval for example.
The flexible members wound by a roller tube system incorporating the drive assembly of the present invention may include shades, screens, curtains or the like that blocks or reflects, or partially blocks or reflects, light. The flexible member may be formed of paper, cloth, or fabrics of any sort. Examples of flexible members include window shades, window screens, screens for projectors including television projectors, curtains that block or partially block entry of light or that reflect light, and curtains used for concealing or protecting objects.
The foregoing describes the invention in terms of embodiments foreseen by the inventor for which an enabling description was available, notwithstanding that insubstantial modifications of the invention, not presently foreseen, may nonetheless represent equivalents thereto.
In the appended claims, the term “flexible member” should be interpreted broadly as including any member capable of being wound that blocks or reflects, or partially blocks or reflects, light. Non-limiting examples of flexible members include shades, screens and curtains.